Two processes, the de novo formation of vessels from differentiating endothelial cells or angioblasts in the developing embryo (vasculogenesis) and the growth of new capillary vessels from existing blood vessels (angiogenesis), are involved in the development of the vascular systems of animal organs and tissues. Transient phases of new vessel formation (neovascularization) also occur in the adult body, for example during the menstrual cycle pregnancy, or wound healing.
On the other hand, a number of diseases are known to be associated with deregulated angiogenesis, for example retinopathies, psoriasis, hemangioblastoma, hemangioma, and neoplastic diseases (solid tumors).
The complex processes of vasculogenesis and angiogenesis have been found to involve a whole range of molecules, especially angiogenic growth factors and their endothelial receptors, as well as cell adhesion molecules.
Recent findings show that at the center of the network regulating the growth and differentiation of the vascular system and its components, both during embryonic development and normal growth and in a wide number of pathological anomalies and diseases, lies the angiogenic factor known as "Vascular Endothelial Growth Factor" (=VEGF), along with its cellular receptors (see Breier, G., et al., Trends in Cell Biology 6, 454-6 [1996] and the references cited therein). EP 0 722 936 discloses certain phthalazines where n is other than 0 in formula I given below, but doesn't disclose their utility against diseases associated with deregulated angiogenesis. DE 1 061 788 discloses a compound with X=oxa falling under formula I below, but no medical use. None of the two discloses any compound of formula I given below wherein n=0 and X is imino or thia.
VEGF is a dimeric, disulfide-linked 46-kDa glycoprotein and is related to "Platelet-Derived Growth Factor" (PDGF). It is produced by normal cell lines and tumor cell lines, is an endothelial cell-specific mitogen, shows angiogenic activity in in vivo test systems (e.g. rabbit cornea), is chemotactic for endothelial cells and monocytes, and induces plasminogen activators in endothelial cells, which are then involved in the proteolytic degradation of extracellular matrix during the formation of capillaries. A number of isoforms of VEGF are known, which show comparable biological activity, but differ in the type of cells that secrete them and in their heparin-binding capacity. In addition, there are other members of the VEGF family, such as "Placenta Growth Factor" (PLGF) and VEGF-C.
VEGF receptors by contrast are transmembranous receptor tyrosine kinases. They are characterized by an extracellular domain with seven immunoglobulin-like domains and an intracellular tyrosine kinase domain. Various types of VEGF receptor are known, e.g. VEGFR-1, VEGFR-2, and VEGFR-3.
A large number of human tumors, especially gliomas and carcinomas, express high levels of VEGF and its receptors. This has led to the hypothesis that the VEGF released by tumor cells could stimulate the growth of blood capillaries and the proliferation of tumor endothelium in a paracrine manner and thus, through the improved blood supply, accelerate tumor growth. Increased VEGF expression could explain the occurrence of cerebral edema in patients with glioma. Direct evidence of the role of VEGF as a tumor angiogenesis factor in vivo has been obtained from studies in which VEGF expression or VEGF activity was inhibited. This was achieved with antibodies which inhibit VEGF activity, with dominant-negative VEGFR-2 mutants which inhibited signal transduction, or with the use of antisense-VEGF RNA techniques. All approaches led to a reduction in the growth of glioma cell lines or other tumor cell lines in vivo as a result of inhibited tumor angiogenesis.
Hypoxia and also a large number of growth factors and cytokines, e.g. Epidermal Growth Factor, Transforming Growth Factor a, Transforming Growth Factor A, Interleukin 1, and Interleukin 6, induce the expression of VEGF in cell experiments. Angiogenesis is regarded as an absolute prerequisite for those tumors which grow beyond a maximum diameter of about 1-2 mm; up to this limit, oxygen and nutrients may be supplied to the tumor cells by diffusion. Every tumor, regardless of its origin and its cause, is thus dependent on angiogenesis for its growth after it has reached a certain size.
Three principal mechanisms play an important part in the activity of angiogenesis inhibitors against tumors: 1) Inhibition of the growth of vessels, especially capillaries, into avascular resting tumors, with the result that there is no net tumour growth owing to the balance that is achieved between apoptosis and proliferation; 2) Prevention of the migration of tumour cells owing to the absence of bloodflow to and from tumors; and 3) Inhibition of endothelial cell proliferation, thus avoiding the paracrine growth-stimulating effect exerted on the surrounding tissue by the endothelial cells which normally line the vessels.
The German patent application DE 1 061 788 names generic intermediates for antihypertensives as belonging to the class of phthalazines. No pharmaceutical use for these intermediates has been declared.